The WEU Assembly has issued a recommendation and memorandum for that issue in 2000, since the ESDP Treaties didn't clear it up.I'm not sure how the three non-EU associate members relate to the ESDP.
As per the Council Treaties (for ESDP), the Associate Members do only have consultation rights within ESDP if NATO assets are used. The European Council also did technically not absorb the WEU in its entirety into the EU, in particular not the cooperation with the Associate Members. The EU Council only absorbed WEU operational aspects.
The WEU Assembly's recommendation is that pretty much ESDP should be modified to accomodate WEU Associate Members.
Afaik, the EU has not acted on this since then really. The WEU has however to my knowledge set up workgroups that "interface" WEU Associate Members with EU/ESDP. There are some restrictions iirc compared to "full members", like no/limited access to EU satellite resources. As of January 2007, the Associate Members still "interfaced" with ESDP like that.